1. Field of the Invention
The present invention relates to a color picture reader that is applied to a color copying device or the like, and in particular, to the correction to the variations in the spectral distribution of the light source.
2. Description of the Prior Art
In the prior art color picture reader, use is made of a light source with high color rendering properties, such as halogen lamp, in order to obtain satisfactory color separation characteristics. In the case of such a light source, the spectral distribution can be maintained constant by keeping the applied voltage at a constant value so that handling of the device is relatively easy. However, such a light source has low efficiency and generates large amount of heat, and hence has been difficult to be applied to a small size device. For that reason, a fluorescent lamp has been examined to replace such a light source.
In the case of using a fluorescent lamp as a light source, its emission efficiency is satisfactory, and moreover, the spectral distribution of the light source can be varied fairly freely by appropriately combining a plurality of kinds of phosphors. Consequently, by combining a fluorescent lamp that possesses such properties with a reader, it becomes comparatively easy to achieve satisfactory color separability. However, the temperature of the tube wall of a fluorescent lamp becomes low when the temperature of the environment is low, and hence, the quantity of emitted light is less and the emissive spectral distribution, too, is different, compared with the rated values (values at the tube wall temperature around 45.degree. C.). In addition, the emission spectrum varies depending upon the length of its use, and moreover, the properties at the time of their manufacture vary widely. Therefore, when a color orginal is read by using such a light source, even with a correction for the brightness, the color separated signals that are obtained will be different from those at the rated conditions. This gives rise to a shortcoming that the truly color separated signals cannot be obtained and that the color signals obtained are different from the actual color orginals.
In attempts to remove these drawbacks, there has been considered such a method as to wait for a fixed length of time, by keeping the light source turned on, after closing the power source or to employ a device in which a heating element is arranged around the fluorescent lamp to control the temperature of the tube wall. However, in the case of waiting for a fixed length of time after turning on the lamp, the time required for stabilizing the light source varies with the temperature of the environment so that it it not satisfactory for stabilization of the light source. Moreover, in the case of controlling the temperature, the control that is given may not necessarily be applied to the section from which the light is actually being emitted. Thus, there exists a drawback that it is not easy to keep the emissive spectral distribution constant. Furthermore, variations in the emissive spectral distribution is a phenomenon which is not peculiar to fluorescent lamps but is common also to many other light sources.